1. Field of the Invention
The present invention relates to a method for teaching a robot capable of implementing a proper torch orientation in performing welding work by play back operation of the robot supporting a torch for arc welding.
2. Description of the Related Art
Where a welding work is performed by play back operation of a robot supporting a welding torch (hereinafter referred to as torch), the orientation of the torch, as well as the position (three dimensional position) of its head, must be properly maintained during the movement of the robot. If the torch orientation is improper, a required welding work can not be accomplished even when the torch head moves on a welding path to trace properly a weld line. Especially, for a weld line path having a large curvature such as corner portion, teaching requiring the torch to sharply change its orientation should be avoided.
FIG. 1 schematically illustrates a conventional method of teaching the position and orientation of a torch to a robot, which is generally utilized. In FIG. 1, a path A→B→C→D→E→F is a welding path along a weld line, in which reference numeral 1 designates a torch mounted on a robot arm (not shown); and reference numeral 2 designates a tool center point set to the head of the torch 1. The tool center point 2 is set with a tool coordinate system of three rectangular X Y Z axes; the origin of the coordinate system is the tool center point 2, and one axis (Z axis) is coincident with the axial center of the torch 1. Thus, it is hereinafter defined that “torch position” refers to the position of the tool center point 2, that is, the origin of the coordinate system, and that “torch orientation” refers to the orientation of the coordinate system. Among many torches shown in FIG. 1, only the torch positioned at a start point A of the welding path is affixed by the reference numerals.
Now, with reference to FIG. 1, a conventional teaching procedure will be explained below.
(1) The robot is advanced by jog feed until the torch position (tool center point 2) coincides with the start point A of the welding path, and then the position of the start point A is taught to the robot. At this time, as torch orientation, an optimum welding orientation (the one shown as a) of the welding path starting from the start point A is taught.
(2) Without changing the taught orientation, the robot is moved along the welding path until the torch position coincides with a position PB located a little before a point B (path junction point B) at which the welding path is bent; and then the position PB is taught. As torch orientation, the orientation a having been taught at the point A is taught as it is.
(3) The robot is advanced further from the position PB until the torch position coincides with the path junction point B, and then the path junction point B is taught. As torch orientation, an intermediate orientation b′ between an optimum orientation (given as b), as a welding orientation of the welding path starting from the start point B, and the orientation a having been taught at the start point A or the position PB is taught.
(4) The robot is advanced further from the point B until the torch position coincides with the position QB located a little beyond the path junction point B, and then the position QB is taught. As the torch orientation, the optimum orientation b as the welding orientation of the welding path starting with the start point B is taught.
(5) Thereafter, the same operations are repeated to teach the positions of points PC, C, QC, PD, D, QD, PE, E, QE, and an end point F, and in addition, as the torch orientation, the orientations b, c′, c, c, d′, d, d, e′, e, e at these points are taught.
The meanings of these codes are as follows:                c: Optimum torch orientation at path CD        d: Optimum torch orientation at path DE        e: Optimum torch orientation at path EF        c′: Intermediate orientation between orientations c and d        d′: Intermediate orientation between orientations d and e        
However, in the above-mentioned conventional teaching method, and the end point F, the position and orientation of the points PB, QB, PC, QC . . . positioned near before and behind the junction points B, C . . . must be taught as well as the start point A, the junction points B, C . . . , so that there is a disadvantage that the teaching work load becomes large. Particularly, it requires a high skill and a long period of time to perform correctly teaching of the torch orientation affecting welding accuracy.
Particularly, the torch orientation at the point at which the welding path is bent, that is, the path junction points B, C . . . must be taught in such a manner that the change in the torch orientation must be smoothly performed in a small section ranging from before to behind these junction points. However, any simple and objective method for finding out such orientation is not available, thereby relying on the intuition and experience of an operator. Therefore, it is difficult to obtain a stable welding accuracy which is not influenced by the skillfulness of an operator.